Method for coating ultrafine particles, system for coating ultrafine particles

ABSTRACT

The invention provides a method for dispersing particles within a reaction field, the method comprising confining the particles to the reaction field using a standing wave. The invention also provides a system for coating particles, the system comprising a reaction zone; a means for producing fluidized particles within the reaction zone; a fluid to produce a standing wave within the reaction zone; and a means for introducing coating moieties to the reaction zone. The invention also provides a method for coating particles, the method comprising fluidizing the particles, subjecting the particles to a standing wave; and contacting the subjected particles with a coating moiety.

PRIORITY

This U.S. Non-Provisional patent application claims priority and benefitas a Divisional of co-pending U.S. Utility application Ser. No.13/366,162, filed on Feb. 3, 2012, the entirety of which is incorporatedby reference.

CONTRACTUAL ORIGIN OF THE INVENTION

The U.S. Government has rights in this invention pursuant to ContractNo. DE-AC02-06CH11357 between the U.S. Department of Energy and UChicagoArgonne, representing Argonne National Laboratory.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the production of coatings onto particles, andmore particularly, this invention relates to the production of conformalcoatings onto particulate substrates.

2. Background of the Invention

Optimization of reaction surfaces is key to more efficient chemicalreactions. The more controlled the catalytic interface, the moreleveraged are the effects of electron confinement.

Well-controlled reaction surfaces would increase efficiencies incatalysis, energy production and energy storage. In the lithium ionbattery market alone, the global U.S. dollar sales are projected todouble by 2016. These sales will be concomitant with the attainment ofhigher efficiencies.

Lithium ion batteries for future plug-in hybrid electric vehicles willrequire high working voltages, on the order of about 5 volts. Theadvantages of these high volt chemistries include the utilization ofrelatively lower cost Lithium- and Manganese-containing spinels, such asLiMn₂O₄, immersed in electrolyte. Manganese systems are less costly andless polluting than cobalt-containing systems.

Problems abound for electrolyte-based systems. For example, structuresphysically immersed into the electrolyte are inevitably attacked by thesurrounding electrolyte though redox chemical reactions. Thissignificantly shortens the battery life and, even worse, createsexplosion hazards.

Also, the sought-after high voltages of advanced battery systems oftenexceed the electrochemical resistance of electrolyte oxidation; as such,irreversible capacity degradation occurs whereby the electrolyte iscompromised. This is due mainly to the high temperatures (more than 55°C.) generated and also due to the electrodes, such as high surface areaspinels, reacting with the electrolyte. Specifically, at target voltagesand the accompanying temperature increases, the manganese in the spinelsplate on the anodes, thereby decreasing recharge capacity.

The use of high surface area materials as reaction surfaces iswidespread, particularly in catalytic sciences. However, inelectrochemical applications, the aforementioned degradation ofelectrodes and electrolyte is problematic. A possible way to preventthis degradation is by coating the aggregate. A myriad of attempts tocoat aggregate include, sol-gel methods and chemical vapor deposition.However, most of these techniques are relegated to situations where thegeometries of the host objects are flat, or large particles, andparticularly, when the uniformity and layer thickness of the coatinglayers are not concerns.

When the size of host particles is reduced to the micro- or nano-scale,physical properties of the coating layer, mainly uniformity andthickness, need to be precisely controlled in order to manipulate ionand electron transport behaviors crossing the interparticle interface.The above-mentioned state of the art coating techniques fall short ofthese objectives.

Generally, the smaller the aggregate, the more valuable it is as areaction surface medium. However, particles with diameters less thanabout 20 micrometers (i.e. microns or μm) show strong flow instabilitywhen placed in a flowing medium, such as a fluidized bed, for mixing.This is due to strong inter-particle forces which exist among ultra-fineparticles. These forces include van der Waals- and electrostatic-forces.Gas tunneling, particle bridging, and clustering during suspension arecommon during mixing and fluidization of ultra-fine particles. FIG. 1Ais a prior art picture of three fluidization beds, demonstratingclustering (on left-hand side), particle bridging (middle), and gastunneling (right-hand side). As a result, fluidizing gas quickly passesthrough the bed via the empty channels. Meanwhile the dense powderregions which comprise the interior bulk of those channel structuresremain static. This “polar structure” phenomenon prevents intimategas-solid contact, the related heat/mass transfer process, and thegas-solid phase reactions sought.

A need exists in the art for a method to economically process ultra fineparticles (i.e., those particles less than about 100 microns indiameter, and more commonly less than 20 microns in diameter). Themethod should eliminate the need for repetitive equipment motion (suchas mixing, shaking or agitating) so as to effectively save energy,thereby allowing for an economical process scale-up. Also the methodshould assure the formation of conformal coatings to the entire surfaceof each of the particles.

SUMMARY OF INVENTION

An object of the invention is to provide a method for coating particlesthat overcomes many of the disadvantages of the prior art.

Another object of the invention is to provide a method for dispersingparticles in a fluid. A feature of the invention is the use of standingwaves to facilitate homogeneous suspension of the particles in thefluid. An advantage of the method is the elimination of isolated regionsof particles, thereby assuring each particle is exposed to the fluid.

Yet another object of the present invention is to provide a device foruniformly dispersing ultrafine particles in gas flows. A feature of theinvention is the use of oscillating fluids as the sole means forhomogeneously fluidizing the particles within a reaction chamber suchthat the energy generated is confined within the resonant (i.e.,reactant) chamber. As compared to the prior art fluidizers of FIG. 1A,the fluidized powders of the present invention homogenously dispersethroughout the reaction chamber as can be seen in FIG. 1B. An advantageof the invention is that the elimination of chamber movement savesenergy.

Still another object of the present invention is to provide a method forcoating porous particles, each particle which is less than about 50microns in diameter and typically below about 20 microns in diameter. Afeature of the method is the use of a standing wave to keep theparticles (including porous materials) homogeneously fluidized. Anadvantage of the invention is that it renders all of the particlesaccessible to coating moieties such that substantially the entiresurface of the porous material is coated to prevent unwanted reactionsof exposed particle surface with the environment of the particle. In anembodiment of the invention, the standing wave comprises the coatingmoieties some of which eventually form the coating on the fluidizedsubstrates.

Another object of the present invention is to provide a conformalcoating onto a substrate. A feature of the invention is that thesubstrate comprises a particle which measures less than about 20 μm indiameter, wherein the substrate is completely encapsulated in an oxidevia atomic layer deposition. An advantage of the invention is that theresulting conformal and pin-hole free coating, with both film thicknessand composition precisely controlled, exhibits superior performancecompared to coated substrates produced via random processes.

Yet another object of the present invention is providing optimizedelectrode-electrolyte interfaces in batteries. A feature of theinvention is that cathodes are coated (no thicker than 20 nanometers,and preferably from about 0.1 to 10 nanometers) with material whichprevents unwanted side reactions such as migration of metal ions. Anadvantage of the invention is the preservation of recharging capacities.

Briefly, the invention provides a method for dispersing particles withina reaction field, the method comprising confining the particles to thereaction field using a standing wave.

The invention also provides a system for coating particles, the systemcomprising a reaction zone; a means for producing fluidized particleswithin the reaction zone; a fluid to produce a standing wave within thereaction zone; and a means for introducing coating moieties to thereaction zone. In an embodiment of the invention, the coating moietiescomprise at least a portion of the standing wave.

The invention also provides a method for coating particles, the methodcomprising fluidizing the particles, subjecting the particles to astanding wave; and contacting the subjected particles with a coatingmoiety.

BRIEF DESCRIPTION OF DRAWING

The invention together with the above and other objects and advantageswill be best understood from the following detailed description of thepreferred embodiment of the invention shown in the accompanyingdrawings, wherein:

FIG. 1A is a picture of the poor fluidization problems of prior artfluidization beds;

FIG. 1B is a picture of a homogenously fluidized bed operating inaccordance with the features of the present invention;

FIG. 1C is a schematic drawing of the invented method, in accordancewith the features of the present invention;

FIG. 2 is a diagram of standing wave assisted fluidized bed reactor, inaccordance with features of the present invention;

FIG. 3 is a diagram of a standing wave fluidized bed reactor usingsynchronized fluid waves, in accordance with features of the presentinvention;

FIG. 4 is a photomicrograph of an alumina-coated cathode, in accordancewith features of the present invention;

FIGS. 5A and 5B are photomicrographs of a zirconia-coated particle, inaccordance with features of the present invention; and

FIG. 6 is an element and weight profile analysis of zirconiumdistribution on a substrate, in accordance with features of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing summary, as well as the following detailed description ofcertain embodiments of the present invention, will be better understoodwhen read in conjunction with the appended drawings.

As used herein, an element or step recited in the singular and precededwith the word “a” or “an” should be understood as not excluding pluralsaid elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising” or “having”an element or a plurality of elements having a particular property mayinclude additional such elements not having that property.

The present invention provides a method for producing functionalizedparticle surfaces. Functionalized surfaces can be classified into twocategories: one is passivation wherein the particle surface is treatedsuch that undesirable reactions and/or related environmental attacks canbe prevented. The other is activation wherein the surface is enabled forvarious desirable reactions to adapt it to other materials, like apolymer, metal, semiconductor and/or ceramic materials. For example,LiCoO₂ or LiMn₂O₄ powders are an ideal cathode material for theapplication of a high efficiency Li-ion battery.

Part of the process for providing functionalized particle surfacesrequires manipulating the particle so as to expose it to reactivemoieties. Particle manipulation is achieved via application of acoherently controlled gas fluidized bed reactor. (Coherent applicationrelates to the insertion of, or the production of, a plurality of waveswithin the reaction zone such that the inserted wave fluid has adirection, amplitude and phase that is similar to that of fluidizedreaction mixture within the chamber, thereby causing interference withthat mixture.) This results in fluidized bed intensification and ensuingsynchronized suspended particles.

Reactor bed intensification facilitates dispersal and general refiningof particles within the range of the nanometer scale, (e.g., the Group Cpowder as defined in the Geldart particle classification). An embodimentof the invention facilitates dispersal and general refining of particlesless than about 100 μm in diameter, preferably less than about 20 μm indiameter, and most preferably between 10 nm and 15 μm. Such ultra fineparticles are used to make cathodes (such as LiMn₂O₄ particles at sizeranges of 5 to 10 microns) and anodes (such as graphite particles havingdiameters of between approximately 5 to 20 microns). Ultra capacitorsmade of metal oxide-coated ultra fine particles (ranging in size fromabout 200 to 500 nm) provide higher energy storage capacity compared toconventional plate capacitors. Also, passivation coating of photovoltaicmaterial on semiconductor nanoparticles results in high efficiency solarcells and LEDs.

The fluidized bed intensification facilitates coating of the particles,such that the coatings (affected via for example atomic layerdeposition) range in thickness from about 1 to 10 nanometers. Theincorporation of these nano-sized reaction centers, particularly atelectrode-electrolyte interface in batteries, optimizes the chemicalstability of the electrode at these interfaces. For example, theinventor found that the resulting thin film resulted in about a 2 to 8fold (as measured by electrochemical impedance spectrometry) increase inLi ion transport across the film, compared to similar sizedelectrode/electrolyte configurations. In addition, the invented coatingmethod provides a means for eliminating secondary or side reactions atthe electrode-electrolyte interface of batteries. Such unwantedreactions include, for example, the aforementioned, catalytically-drivendecomposition of electrolyte.

The present invention provides a process for encapsulating electrodegrains with either an inorganic or an organic phase so as to prevent theaforementioned electrolyte breakdown, particularly at the positiveelectrode interface. An exemplary application of this process is theencapsulation of the spinel LiMn₂O₄ with metal oxides such as ZrO₂ orAl₂O₃ so as to minimize the surface area of the active material incontact with the electrolyte. A means for coating the spinel is atomiclayer deposition (ALD).

The encapsulation facilitated by the present technique results incontinuous coatings over substantially the entire surface of the spinelsuch that no gaps exist. The continuous coating isolates the spinel sothat no direct physical contact occurs between the spinel and theelectrolyte or no fluid communication exists between the surface of thespinel and environment. The conformal metal oxide coatings (preferablycomprised of ionically-conductive coating materials) thus formed alloweasy diffusion of Li-ions, yet are thin enough to allow electrontransport. (Conformal coatings are those in which the coating is uniformacross the surface of the particle so that the surface shape of thecoated substrate closely resembles that of the underlying substratesurface.)

The coatings prevent Mn ions from leaving the cathode and thereforeprevent cathode distortion. Some coatings generated by the inventedprocess slow Mn ion migration from the cathode, thereby allowing Li ionsto reach the anode first. The invention enables the layer thickness tobe tuned, such that coating thicknesses are less than about 20 nm,preferably between about one half a nm and 20 nm, and most preferablybetween about 1 nm and 5 nm. In one embodiment, Li ions transport acrossthe film which is coating the cathode, contrary to a conventionalself-generating film on the same electrode.

FIG. 1C provides a schematic diagram of the invention for deepgas-fluidized bed reactors. (Deep beds are defined as having a bedheight to column diameter ratio greater than 1.) Generally, particles tobe coated are suspended within the confines 13 of a housing. A means forproviding particle suspension includes a standing wave assistedgas-solid flow field 15. In an embodiment of the invention, theparticles are relegated at a pressure node 17, defined as where thelocal wave pressure 23 and levitation force 25 of the gas-solid liquoris zero, while the local wave velocity 27 is maximal. This region of thewave field enables stable levitation. This satisfies Bernoulli'sprinciple of energy balance. Because the levitation force 25 acting oneach particle depends on this particle's location, and is directedtoward the nearest pressure node, 17, 21, the region of the wave fieldnear the pressure node enables stable particle levitation. Particularly,the radiation or levitation force is much larger in standing than inrunning waves.

Other pressure nodes 21 within the wave field flank the stable wavefield pressure node 17. Levitation forces 19 act in the direction ofthese flanking pressure nodes to produce stable levitation by upward ordownward flow.

A vortex structure (not shown) is generated locally, due to unevenpressure distribution, when enough particles are present.

The inventor has discovered a sequence of well-defined transitionsbetween disordered states dominated by bubbling instabilities andordered periodic sub-harmonic patterns as the frequency, the amplitudeof modulation and the fluid flow rate are varied. These transitionsoccur for both deep beds and shallow beds. The observed ordered statesfor the shallow bed include periodic squares, stripes and some morecomplicated structures. The pattern wavelength is determined by thefrequency of the modulation of the mean flow rate, as further discussedin J. Li, I. S. Aranson, W. K. Kwok, and L. S. Tsimring, Phys. Rev.Lett. 90 (2003) 134301, the entirety of which is incorporated byreference. The transition to periodic patterns is associated with adecrease of the gas pressure difference across the bed, suppressing theformation of large gas bubbles.

Specifically, the inventor found that when a fluid flow is superimposedon a granular bed layer, wherein the frequency of the fluid flow issubstantially similar to the fluidization frequency of the bed layer,pattern formations in the bed layer occur. The flowing fluid (such as aninert gas like nitrogen or argon) contacts the bed layer at the bed'snatural undulating rhythm to provide particle dispersion whileconcomitantly minimizing interparticle collisions.

The invention provides a new methodology to disperse particles in a gasflow, which is based on the spatial/time regulation function of astanding wave of pressure that can be generated by resonance between amodulated gas flow and its reflected wave from a reflector installed atthe top of the bed. A uniform gas fluidization of ultrafine powders isachieved as well in a deep fluidized bed by implementing the inventedmethod.

Gas Fluidized

Reactor Detail

Generally, an exemplary device to facilitate the invented particlecoating method comprises a housing. A first end of the housing isadapted to receive compressed gas. As such, proximal to and interior ofthat first end is constructed a porous plate 14 to provide anapproximately uniform gas distribution. In an embodiment of the device,the porous plate is covered by thin metal mesh with the hole sizesmaller than the diameter of the granular particle. This mesh sizeprovides a means to prevent the fluidized particles from clogging theplate. However, if the plate is sintered, no mesh is required inasmuchas the inventors have found that sintered plates provide well-controlledpore sizes. If the right grade of pore size is selected, the particles(which often exist as clusters) will not plug the pores. For example,the 10 micron diameter pores of sintered plates have never pluggedduring inventors' experiments. Particles to be coated are placed abovethe plate, and above the mesh in instances where mesh is also used.Optionally, packed rings are randomly situated below the plate for flowpre-distribution. These rings prevent gas from ejecting directly to thecenter of the gas-fluidized bed reactor. In an embodiment of theinvented device, rings are piled on top of each other to form zig-zaggedpassageways to serve as a means for uniformly distributing thefluidizing gases before the gases contact the plate. The rings arecomprised of any material such as glass or ceramic or stainless steel,or any other substance inert to the precursors used in the coatingprocess.

Dry compressed gas is pumped through the particle layer, and the flowrate is controlled by a proportional valve with a very short responsetime. The flow velocity is periodically modulated using the proportionalvalve. The fluidization efficiency is monitored by measuring thepressure drop across the bed and/or by analyzing the surface modulationof the bed.

An embodiment of the invented coherently-controlled gas-fluidized bedreactor is depicted in FIG. 2 and comprises the following:

-   -   A housing 12 of the reactor, such as is a column or sleeve        composed of stainless steel, quartz, or some other inert        substrate, so as to define the physical confines of the reaction        zone. Specifically, the housing 12 serves to confine a fluidized        bed with the interior 13 of the housing.    -   A gas distributor 14 is positioned within the housing, and near        a first end of the housing. The distributor comprises a means        for regulating ingress and egress of compressed gas to the        interior of the housing. One such means is a substrate or        webbing with a predetermined mean pore size, (e.g., about 10        microns) along with a top extension bed region as a means to        slow down incoming gas. In an embodiment of the invention, a        depending end of the housing, shaped as a cylinder, is        constructed of sintered porous metal plate to provide an        approximately uniform gas distribution. Optionally, the porous        plate is covered or otherwise overlaid by thin metal mesh having        a hole size smaller than the diameter of the granular particles        comprising the fluidized bed. This serves as a means to impede        the particles from clogging the underlying plate.    -   A plurality of porous filtering tubes 16 is installed at the top        of the reactor. The tubes provide a means of egress of the        reaction and/or product gases while simultaneously providing a        means for keeping the target powder within the reactor. An inert        gas is periodically injected in a back blast, counter current        fashion through the tubes 16 to remove any powder adhering to        the tubes or filters positioned within the tubes.    -   A plurality of apertures and associated conduits 26 are formed        along longitudinally extending walls of the reactor and in close        spatial relationship to the portions of the reactor confining        the bed. These apertures provide a means of ingress (situated at        an upwardly extending portion of the reaction chamber) and a        means of egress (situated at a depending, i.e., a downwardly        extending portion of the reaction chamber) of the powder to and        from the reactor. The accompanying arrows shown proximal to        elements 26 indicate the direction of particle flow. However,        embodiments can exist where the particle flow is reversed. The        apertures 26 facilitate loading and evacuation of the powders        over the gas distributor, in either a batch or continuous mode.    -   A wave reflector 18 is positioned above the surface of the        powder bed. This reflector, comprised generally of metal, glass,        ceramic, plastic, or some other inert material (relative to the        precursor moieties) defines a surface to reflect a wave        impinging upon in. In an embodiment, the reflector is flat and        elliptical in shape. The reflector provides a means for        generating a standing wave of pressure. The reactor is        configured so as to confine the standing wave between the gas        distributor 14 and the wave reflector 18, the entire wave and        its associated energy therefore confined to the interior 13 of        the housing. The position of the wave reflector is adjustable        from a remote location (e.g., from outside of the fluidized bed        reactor) to optimize the wave field generated. In an embodiment        of the invention, a means 20 for adjusting the wave reflector        comprises a rigid substrate 20 attached to the wave reflector,        the substrate comprising regions 22 adapted to be manipulated by        hand, chuck or other mechanical means.        Standing Wave        Detail

The invention provides for spatial and/or temporal controlling of adynamic process by using a modulating standing wave. A standing wave isa stationary wave that oscillates in time, but has a spatial dependencethat is stationary, per the algorithm: sin(2πx/λ). Standing waves arecreated when the medium is moving in the opposite direction to the wave,or it can arise in a stationary medium as a result of interferencebetween two waves traveling in opposite directions, such as the standingwave generated through interaction of the original wave with itsreflected wave. In the second case, for waves of equal amplitudetraveling in opposing directions, there is, on average zero propagationof energy. This feature is very useful and different from that for atravelling wave. Particularly, the self regulation function of standingwaves in both the time and spatial domains provides a means forrelegating primary powders to stay within multiple cells where strongself-generated vortex occurs and stirring naturally forms. This uniqueflow structure allows for the generation of uniform powder dispersionand/or smooth powder fluidization.

In an embodiment of the invention, initiation of a standing wave occurswhen a superimposed oscillatory gas flow is injected into agas-fluidized bed. In an embodiment of the invention, a single gas flowis utilized, such as nitrogen gas, the flow comprising a constant partand an oscillatory part. These parts are injected into the particle bedsimultaneously. A means for generating the constant part and oscillatorypart of the gas flow comprises a combination of a digitally controlledproportional valve and a signal generator. The signal generator producesvarious electrical waves which are used to control the proportionalvalue. Two components comprise the signal: a direct current (DC)component which supplies constant charge to the valve (and therefore aconstant gas stream flow), and an alternating current (AC) componentwhich varies with time to provide a pulsed or oscillatory electricalsignal to the valve. This pulsed component generates the oscillatoryportion of the gas stream. An exemplary algorithm for this two componentelectrical signal is as follows:A+B×sin(Ω×T)wherein A is the DC component, B is the AC component, Ω is the amplitudeof the sine wave and T is the time. The inventors have found that byvarying A, B and/or Ω, the constant and oscillatory portions of thefluidized gas are varied to optimize particle fluidization and coating.

The standing wave is generated and confined between two fixed positions.As such, the energy embodied in the standing wave is focused within themixing chamber and not transferred outside of the chamber.

The standing waves occur between two fixed positions, such as opposingsides of a reaction vessel. One way in which a standing wave isgenerated is when two waves, with identical frequency, travel inopposite directions in a stationary medium as a result of interference(e.g., reflected waves from walls or the ceiling and/or reflector of thereaction chamber). A schematic of this method is found in FIG. 2. Theparticles fill the entire bed space, with more particles staying at andnear the nodes 19. Generally, the nodes are located at the intersectionof adjacent pressure waves. In this instance, energy is focused withinthe bed for fluidizing particles between gas distributor and the wavereflector, rather than transporting particles across the entire reactor.A clear freeboard condition develops indicating that energy is focusedwithin the bed for fluidizing particles, rather than transporting theparticles across the entire reactor. The inventors have found that thisgeneration technique is good for controlling ultra fine C powders, i.e.,those particle with diameters of approximately 20 microns or less.

Another embodiment of the invention generates a standing wave bysimultaneously introducing two oscillatory flows with the same frequencyfrom both the bottom and the top bed. This scenario is schematicallydepicted in FIG. 3. The advantage of this method is that the intensityof the standing wave formed can be larger than that formed when just oneoscillatory flow is utilized with a reflecting surface.

Another way that standing waves are generated is when the particlemedium moves in the opposite direction to a wave caused by the insertionand superimposition of inert fluid into the reaction chamber. Thisscenario results in improved particle suspension. The inventors foundthat this technique is good for controlling B/D powders, i.e., thosepowders of approximately 200-500 microns in diameter (B size) and morethan 1000 microns in diameter (D size). The modulated fluid can be sine,square and other oscillatory fluid waves. These waves are generated inmyriad fashion, for example by using a magnet- orelectrically-controlled proportional valve or similar metering system.

Also, the waves can create vortex structures. This is because when awave travels through a viscous medium, it interacts strongly with thismedium, by which energy can be effectively transferred onto the medium.The inventors have found that for their invented gas-solid system, anincreasing particle density results in system viscosity increases. Whenthe resulting fluid is excited by a standing fluid wave, an unevenpressure distribution develops, resulting in a circulating patterned(vortex) structure.

The standing waves exhibit higher intensity (i.e., higher amplitudecompared to the intensities of its originating waves. The establishmentof standing waves indicates that the measured bed-pressure dropapproaches the suspended particle bed weight. This causes a morehomogenous suspension of substantially the majority of the particleswithin the reaction zone, concomitant with continual mixing of theparticles (see FIG. 1A).

An advantage of using standing waves is that the energy is focused onthe media comprising the wave, and not transferred to outside of themedia. The antinode portion of the wave moves more than the node portionof the wave. Relative to the antinode portion, the node portion remainsfixed within the wave medium. This is because at pressure nodes, thepressure is fixed. However, at the anti-nodes, pressure wave changesperiodically between the maximum positive and negative peak pressures.It is due to this pressure difference between the node region and theanti-node region that creates conditions leading to vortex formation.The distance between two conjugative nodes or anti-nodes is λ/2.

The inventor has determined three mechanisms for generating a secondwave from which standing waves result:

1. Exploitation of naturally-existing pressure waves in the axialdirection in a gas-fluidized powder bed.

2. Reflecting waves in the radial direction (relative to the incidentwave) reflective wave originating from the fluidized bed reactor wall.

3. Back reflecting the initial wave using a specially designedreflector, the back reflection traveling in the axial direction alongthe initial flow path.

Standing waves provide a pulsed power effect to the particles. In anembodiment, the standing wave fluid comprises compressed, pulsednitrogen gas. However, other relatively inert gases are also suitable,including argon, neon, helium, and combinations thereof. The provisionof pulsed fluid obviates the need for extra power sources or equipmentto maintain the homogeneous dispersion of particles within the reactionchamber.

The distance between gas distributor and the wave reflector should bethe integer times of half of the wavelength. Since gas flow velocity isdetermined mainly by particle fluidization and the wavelength needs tobe shorter than the bed height to form a standing wave, the excitingfrequency can be accordingly determined. As such, it is systemdependent, normally in a low frequency (i.e., less than about 100 Hz,typically between about 1 and 50 Hz, preferably about 10 Hz) range.

The inventor has established a sequence of well-defined transitionsbetween disordered states dominated by bubbling instabilities andordered periodic sub-harmonic patterns as the frequency, the amplitudeof modulation and the fluid flow rate are varied. The observed orderedstates include periodic squares, stripes and some more complicatedstructures. The pattern wavelength is determined by the frequency of themodulation of the mean flow rate, as elaborated in J. Li, I. S. Aranson,W.-K. Kwok, and L. S. Tsimring, Phys. Rev. Lett. 90 (2003) 134301, theentirety of which is incorporated herein by reference. The transition toperiodic patterns is associated with a decrease of the gas pressuredifference across the bed, suppressing the formation of large gasbubbles.

Particularly, the inventor found that it is the superimposed flow thatinterferes with the granular bed layer when its frequency concurs withthat of the bed layer, leading to pattern formation. For thenonequilibrium system of fluidized solid particles, there always existsa unique flow regime for which certain frequencies dominate. A cohesive(or adaptive) controlling strategy utilizes the bed's natural rhythm,making possible a high bed expansion, and thus improved particledispersion and significantly suppressed interparticle collisions, yetwith a much lower fluid or energy input. The bed's natural frequency canbe measured or calculated from the literature such as H-T Bi, Chem. Eng.Sci. 62 (2007) 3473, the entirety of which is incorporated by reference.

This coherently-controlled gas fluidization technique eliminates theneed to rotate, jostle or otherwise move the reaction chamber. Rather,dispersion is achieved by contacting the particles to a standing wave.This results in homogeneous fluidization of the particles achieved atlow energy and equipment costs. As such, the fluidized powder bed iskept moving solely by the applied fluid wave.

The inventor found that periodic variation of the pressure wave canoccur by interfering with the reflected wave originating from thereflector surface, such as a side wall. This periodic variation leads toincreased pressure fluctuations, and provides a means for preventingparticle agglomerate formation. Homogeneous fluidizations of ultrafinepowders in shallow beds are enabled. For the purposes of thisspecification, shallow beds have bed heights less than their reactor'sdiameters (H>D). In some embodiments, shallow bed heights are typicallyno more than 5 centimeters, such that the wall effect can be neglected.

With increasing bed height, where H>D, the inventors found that thewall-induced reflecting wave effect diminishes. In these instances, thestanding wave is generated utilizing a wave reflector installed abovethe bed.

A modulated pressure-wave generator is used to drive the gas-solidsuspension into a resonant standing wave state in the gas-fluidized bedregion. An embodiment of the generator comprises a signal generator,amplifier, proportional valve and mass flow meter. The system produces amodulated oscillatory gas flow of fluid to to coherently fluidizeprimary powders in the gas-fluidized bed.

ALD Detail

The gas-phase surface adsorption of ALD precursors is a very rapidprocess and its rate-limiting step is determined by the contact chanceof a precursor molecule locating a vacant nuclear site and reacting. Instate of the art coating protocols, the aforementioned particleinteractions resulted in protracted times for precursor molecules toreach their destinations, mainly because they are transported bydiffusion through a torturous pathway. Heretofore, mass transfer hasbeen very limited.

A precursor dosing system preferably includes dedicated lines for eachselected precursor to the gas fluidized bed reactor. Each line can becomposed of a mass flow controller, a precursor bubbler to hold therequired reactant, ALD values located both ahead and after each bubblerto control precursor introduction, and a needle valve or vacuumcontroller located downstream of the bubbler to adjust outlet pressure.A bubbler-based dosing system is preferred for precursors with vaporpressures above about 1 torr at room temperature, so as to have adequateprecursor exposure to assure conformal film growth.

For precursors having low vapor pressures (e.g. below about 1 torr),direct-liquid injection systems are preferred in which the liquidprecursor is directly injected into a mixing/evaporation chamber forvaporization before entering the fluidized bed reactor.

Salient features of the invented system in which ALD is utilizedincludes the following:

-   -   A fluidized bed reactor, including a gas distributor/powder        separator and the coherently controlled pulsing flow system.    -   A chemical precursor dosing system, such as a        Labview®-controlled configuration.    -   Middle-level vacuum system (approximately 1 Torr) under an        operating fluidization condition of about 3U_(mf).    -   System for monitoring coating processes and various species        concentrations (such as on-line mass spectroscopy).    -   Temperature control system.

A gas-solid system operating under the middle-level of vacuum comprisesa vacuum pump (Edwards Model: E2M80FX), chemical/particle traps,controlling valves, multiple vacuum transducers. The purpose of filmcoating under a vacuum condition is two fold: 1) to confine hazardprecursors within the system, and 2) to facilitate film growth asmolecules has good diffusion and flowability under such a level ofvacuum, typically 0.001 torr<P<100 torr.

The ALD reaction typically occurs under a relatively low temperature, ascompared to its CVD coating process. The gas-fluidized bed reactor needsto be heated to a certain temperatures, e.g. 500° C., 1) to removemoisture and volatile organics from the powder surfaces, and, e.g. 180°C., 2) to maintain the ALD reaction. Also, the precursor deliveringlines are also needed to be heated with a temperature about 10° C.higher than that for the precursor bubbler to prevent any precursorcondensation in the line.

The ALD coating process is a gas-phase reaction. Both reactants andproducts are in the gaseous type except the coated thin film.Preferably, a mass spectrometer provides an on-line and real-timemeasure and monitor of various gas species during the entire ALD-coatingprocess. The partial pressure of the precursors also indirectly providesnecessary information on whether primary powders expose enough.

ALD Precursor Injecting

System Detail

An embodiment of the precursor injection system includes a plurality ofmeans of ingress of the reactant moieties into the fluidized bedconfines. FIG. 2 depicts two conduits 24, to provide the selectedprecursors to the gas-fluidized bed reactor. Each line can be composedof a mass flow controller, a precursor bubbler to hold the requiredreactant, two ALD valves located both ahead and after each bubbler toconduct open/close controlling, a needle vale or a vacuum controllerlocated in the down streaming to adjust the outlet pressure; Such abubbler-based dosing system is suited for precursors with high vaporpressure, e.g. >1 torr at room temperature. Otherwise, the partialpressure, or concentration, of the precursor would not be high enough toallow for a good film growth.

For precursors with low vapor pressures (ranging from about 0.001 to 1torr at room temperature), a direct-liquid injection system is requiredby which the liquid precursor is directly driven into amixing/evaporation chamber, and then it is vaporized before it entersthe fluidized bed reactor. An adequate (based on theoretical massbalance of a certain volume or weight of particle loading) supply of aprecursor is preferred since a significantly large amount of particlesurface exists in a gas-fluidized bed. In continual operation, acontinual supply of precursor is suitable.

The typical procedure for conducting an ALD-coating in acoherently-controlled gas-fluidized bed include: 1) surfacepre-conditioning of primary particles by annealing particles at a hightemperature of approximately 500° C. under vacuum for about 1 to 2 hoursto clean any volatiles and/or free water that may be physically adsorbedon particle surfaces; 2) surface treatment of the primary particles bye.g. using O₃, to make available the required functional group, e.g.—OH, for initializing the ALD film growth; 3) alternative dosing of twochemical precursors in a six-step sequence:

a) precursor 1, e.g. Al(CH₃)₃, or Zr[N(CH₃)(C₂H₅)]₄

b) inert gas (e.g. N₂) purging,

c) a short period of vacuum exposure (no gas entering),

d) precursor 2, e.g. H₂O,

e) inert gas N₂ purging, and

f) a short period of vacuum exposure (no gas entering).

Among the steps, inert gas purging and vacuum exposure are specificallydesigned to make sure that no former precursor would remain within thesystem, thereby preventing any CVD-like deposition since a CVD-type ofdeposition deteriorates the quality of the ALD ultrathin film.

Controlling of the precursor dosing is conducted via a series of gasactivated diaphragm valves, which are automatically executed by using anALBVIEW computer program. Various operating parameters can all bespecified in the program, including flow rate of carry gases, dosingtimes, interval purging periods, vacuum exposure times, and totalcoating cycles. In addition, a gas-activated diaphragm valve is alsoinstalled for each precursor dosing line at the entrance of thegas-fluidized bed reactor to seal the precursor line when it is duringits “off” period such that a potential inter-precursor contamination canbe prevented. Also, this valve is synchronized with the other two ALDdiaphragm valves in the same precursor line. Moreover, a needle bellowvalve is installed as well in the downstream of the bubbler for eachprecursor line to regulate the outlet pressure.

High purity N₂ from a pressurized gas cylinder, which acts as bothfluidizing and a purging gas, is introduced into the gas-fluidized bed.A mass flow meter (e.g., an Aalborg, Model GFMS series) is used tomeasure the N₂ flowrate. A proportional valve that is electronicallycontrolled by a function generator (e.g., a Textronix Model AFG3000) isused to generate flow/pressure fluid modulation such as gas modulation.Gas modulation is generated by any pulsing generation methods (for afluid), such as a gas pump, or using a gas modulation system composed ofgas cylinder, proportion valve, and function generator. Such a modulatedflow is maintained in the gas-fluidized bed reactor for the entirecoating process except the short periods of vacuum exposure in step cand f in the sequence, in which case the modulated flow is switched tobypass the reactor. A well-defined modulated gas flow is achieved bymanipulating the gas cylinder outlet pressure or inlet fluidizingpressure via the fluid modulation system as described supra.

As the intensity of a pressurized gas can be much stronger, andstraightforward than that generated by a sound (acoustic and ultrasonic)source or a mechanical vibration, a modulated fluid wave is capable ofgenerating a much more effective modulated pressure filed in a modulatedgas-fluidized bed. When the modulated pressure wave approaches thereflector in the fluidized bed, it is reflected back to resonantly reactwith the original wave to form a standing wave. As a result, the flowmodulation can be further enhanced, enabling to effectively regulate thefluid-particle system in both spatial and temporal domain to form ahighly homogenous two-phase flow field, or the uniform particledispersion in a gas flow.

In essence, compressed fluidizing gas provides the energy to overcomeinterparticle forces and gas channels, thereby allowing the reactor bodyto remain motionless. These forces include electrostatic interactionsand van der Waals forces.

Another embodiment of the invention utilizes acoustic-assistedfluidization through levitation.

The invented method is applicable to enable powder-related applicationswhere uniform particle dispersion is required. For example, the methodfacilitates the production of conformal surface coatings of granularmaterials via atomic layer deposition in a gas-fluidized bed reactor.This will enable large scale production (i.e., batches of 500 gramsinitially, and 5 kilograms with further scale up) of functionalizedparticles for use in Li-ion batteries, super capacitors, catalysts,nanocrystal solar cells, and other applications where enhanced surfacearea between catalyst and reactants are crucial.

In an embodiment of the method, a modulated superimposed fluid wave iscoherently applied to a gas-fluidized powder bed. The fluid wave isintroduced in the reaction environment upstream from the ingress pointof the reactants. At the downstream end of the environment, a wavereflector is installed in the flow path of the fluid wave.

The provision of the wave reflector facilitates the formation of theaforementioned standing wave. In an embodiment of the invention, thestanding wave is generated by interference between a traveling wave andits reflective wave. The standing wave generated thereby confines thefluidized particles to particular zones of the fluidized bed. Generally,the standing wave is generated by triggering resonance with the system'sboundaries, such as a reactor side wall or a top reflector surface.Alternatively, a standing wave can be generated by inducing resonancewith the complex fluid system itself in situations where a naturalfrequency within the fluid system either interferes with multiple wavestraveling in opposite directions within the system or else the naturalfrequency results from fluid moving in the opposite direction to theimposed wave.

Particle Detail

A wide variety of particulate materials can be used as the hostsubstrate. In this invention, dielectric, semiconductor and metalmaterials are the most typical base materials. The base powder materialincludes metal, semiconductor, and dielectric and ceramics, e.g. Li—Mn—Ospinel cathode and its composite materials. The sizes of such powdersare preferably less than 100 microns; more preferably is less than 20microns.

Exemplary host materials include: 1) lithium transition metal oxides andgraphite/graphene that are used for Li-ion battery electrodes; 2)dielectric materials for super-capacitors; 3) semiconducting materialsfor the next generation of solar cells and LEDs; and 4) metals for aspectrum of chemical reaction catalysts as applied typically inpetroleum refining, biomass pyrolysis and many other areas. Otherrelated base materials can be nano-constructed (wire, tube or particle)composite materials, like carbon fiber reinforced plastic materials. Allgranular materials, in general, that need surface modification to have anew function are all in the category of this invention, acting as basematerials for such ALD-coating technology.

The size of the host particles largely depends on the particularmaterial and its specific application. The particle size suited can beup to 100 microns, e.g. typically for catalyst applications, withpreferred particle sizes ranging from 0.1 to about 10 microns typicallyfor Li-ion battery applications, and more preferred particle sizesranging from 1 to about 100 nm, e.g. for the next generation ofphotovoltaic cell or LEDs applications, and the most preferred particlesizes ranging from about 1 to 60 nm, or less than a specific dimensionas defined by Bohr diameter of that base material, e.g. the exciton Bohrradius for PbSe is 46 nm. This size range is of particular interestsince the materials constructed thereby exhibits a lot of new andexciting physical properties and unprecedented functions, leading to newmaterial discoveries and technology breakthrough. Moreover, thisALD-CCGFB coating process can also be applicable to particles havingdiameters more than 100 microns.

Fluidization Example 1

Li-ion cathode particles, having a means size of between about 5 and 10microns, were homogenously fluidized. The tap density was about 2 to 2.8g/cm³. (Tap density is the powder density measured after the powders aretapped for a certain period of time while residing in a graduated cup,cylinder, container or other volume measuring means. The wave frequencywas about 3 Hz using about 12 pounds per square inch (psi) inlet N₂pressure and atmospheric bed pressure. The fluidization occurred in alaboratory-scale sized fluidized bed having approximately a 4 centimeteroutside diameter. Both atmospheric and vacuum conditions producedsuitable alumina and zirconia coatings, as depicted in FIG. 4 and FIGS.5A and 5B, respectively, and discussed supra. Stable fluidizationendured until the wave media feed was stopped.

Fluidization Example 2

LiNi_(0.5)Mn_(1.5)O₄ particles of approximately 5 to 10 microns werefluidized. The wave frequency was about 3 Hz. Approximately 12 psinitrogen pressure and a vacuum bed pressure (2 torr) was utilized.(Typically, frequencies between about 1 and 10 Hz are preferred.) Theinventor discovered that stable fluidization occurs under bothatmospheric and vacuum conditions for several hours until the gas flowis terminated.

Example 3

Ultrafine particles are also partially homogenized via ultrasound waves.In this example, iron powder between about 1 to 10 microns in particlediameter are fluidized using 250 kHz frequencies, and at about 170 dB insound intensity. This sound intensity represents the amplitude of theultrasonic wave. Generally, between 100 dB and 200 dB are suitableacoustic frequencies for ultrasonic mixing of the particles. Compared tofluid modulation discussed supra, the sound wave approach imparts lowerpressure intensities.

In all examples, a wave reflector is utilized. Also in all examples,evidence of homogenous dispersion was an observed by either visualizingthe powder fluidization (by looking through the walls of the reactorcontainer when quartz reactors are utilized), by noting pressures dropacross the fluidized bed, (which result from a suspended bed weight), bycalculating the suspended particle density, or a combination of thesemeans. The process continued until the wave fluid was terminated. Thiscompares to a relatively short fluidization time seen in conventionalbed reactors (which do not employ standing waves) due to the lack ofparticle surfaces.

The invented fluidization technique is used to enable ALD-coating ofmetal oxides on Li—Ni—Mn—O particles. In an embodiment of the invention,the ALD precursors are introduced upstream from where the wave fluid(such as nitrogen gas) is introduced. In another embodiment, the ALDprecursors are introduced downstream from where the wave fluid isintroduced.

Coating Film Detail

A variety of ultra-thin coatings can be deposited onto the surfaces ofthe primary particulate materials. The thin coating material includesmetal, nitrides, and metal oxides, e.g. ZrO₂, and Al₂O₃. Thickness ofthe coated layer is between about 0.1 and 100 nm, preferably betweenabout 0.1 and 10 nm, and most preferably more than 0.2 nm but less thanabout 3 nm. Some exemplary coating materials and the related precursorsare shown in Table 1.

TABLE 1 Coating Candidates for particle encapsulation. Depositedmaterials Precursor Al₂O_(3,) AlN, Al Aluminum sec-butoxide Aluminumtribromide Aluminum trichloride Diethylaluminum ethoxideTris(ethylmethylamido)aluminum Triethylealuminum TriisobutylaluminumTrimethylaluminum Tris(diethylamido) aluminum Tris(ethylmethylamido)aluminum MgB₂, BN, B, Diborane [B₂H₆, gas] Trimethylboron [(CH₃)₃B, gas]B₄C, B₂O₃, B doping Co, CoO, Bis(N,N′-Diisopropylacetaminato)cobalt(II)Dicarbonyl(cyclopentadienyl) CoSi2 cobalto(I) Cu, CuO(N,N′-Diisopropylacetaminato)copper(II) CaF₂ LaF_(3,) TiF4 TaF5 NH4FSrF_(2,) ZnF_(2,) MgF₂, YF3 Fe, FeOBis(N,N′-di-tert-butylacetamidinato)iron (II) Ga2O3, Ga, Galliumtribromide Gallium trichloride Triethylgallium Triisopropylgallium GaN,GaP, Trimethylgallium Tri(dimethylamido)gallium Tri-tert-butylgalliumGaAs Ge, GeO₂, Digermane Germane Tetramethylgermanium GeSi HfO2, Hf3N4Hafnium (IV) chloride Hafnium (IV) tert-butoxideTetrakis(diethylamido)hafnium (IV) Tetrakis(dimethylamido)hafnium (IV)Tetrakis(ethylmethylamido)hafnium (IV) In₂O₃, InN, Indium trichlorideIndium(I) iodide Idium acetylacetonate Triethylindium InP, InAs, IndiumTin oxide GaN, Si₃N₄, N,N-dimethylhydrazine Ammonia AzidotrimethylsilaneInGaN, AlGaN MgO Bis(ethylcyclopentadienyl)magnesium Nb2O5 Niobium (V)chloride Niobium (V) ethoxide Ni, NiO Bis(methylcyclopentadienyl)nickel(II) InP, GaP Phosphine Tert-ButylphosphineTris(trimethylsilyl)phosphine Pt, PtO2Cyclopentadienyl(trimethyl)platinum (IV) Ru, RuO2Bis(ethylcyclopentadinyl)ruthenium (II) Sb source TrimethylantimonyTris(dimethylamido)antimony SiO₂, Si₃N₄,2,4,6,8-tetramethylcyclotetrasiloxane Dimethoxydimethylsilane DisilaneSiC Methylsilane Octamethylcyclotetrasiloxane SilaneTris(isopropoxy)silanol Tris(tert-butoxy)silanolTris(tert-perntoxy)silanol Ta2O5, TaN Pentakis(dimethylamido)tantalum(V) Tantalum (V) chloride Tantalum (V) ethoxideTris(diethylamino)(ttert-butylimido)tantalum (V) TiN, TiO2Bis(diethylamido)bis(dimethylamido)titanium (IV)Tetrakis(diethylamido)titanium (IV) Tetrakis(dimethylamido)titanium (IV)Tetrakis(ethylmethylamido)titanium (IV) Tatanium (IV) bromide Tatanium(IV) chloride Titanium (IV) tert-butoxide V2O5 Vanadium (V)oxytriisopropoxide W, WO₂, WO₃, WCBis(tert-butylimido)bis(dimethylamido)tungsten (VI) Tungstenhexacarbonyl Tungsten (VI) chloride Tungsten (VI) fluoride Y2O3,Tris(N,N-bis(trimethylsilyl)amide)yttrium (III) Yttrium (III)butoxideYBaCuOx ZnO Diethylzinc Zr3N4, ZrO2 Tetrakis(diethylamido)zirconium (IV)Tetrakis(dimethylamido)zirconium (IV)Tetrakis(ethylmethylamido)zirconium (IV) Zirconium (IV) bromideZirconium (IV) chloride Zirconium (IV) tert-butoxide

The inorganic deposits formed in the ALD process may take the form ofindividual particles, or a continuous film. The physical form of thedeposits will depend on factors such as surface pre-treatment of theprimary particles, the physical form of the substrate, and the number oftimes the reaction sequence is repeated.

For ALD coating, three physical parameters are unique and essential to agood film quality: firstly, the deposits of inorganic material are“ultrathin”. “Ultrathin” is defined herein as a thickness of the depositup to 50 nm, preferably from about 0.1 to about 10 nm, and mostpreferably from about 0.5 to about 3 nm (i.e. monolayer and/or a fewatomic layers). This can be done by simply specifying the cycle numberof the deposition.

Secondly, the deposits of inorganic material preferably form a“conformal” coating. “Conformal” is defined herein as the thickness ofthe coating being relatively uniform across the surface of the particle,so that the surface shape of the coated substrate closely resembles thatof the underlying substrate surface. Conformality can be determined bymethods of direct observation, such as AFM, or transmission electronspectroscopy (TEM) that have resolution of 1 nm or below. The desiredsubstrate surface is preferably coated substantially without pinholes ordefects. This can be controlled by 1) self-termination for the halfreaction and 2) enough exposure to the precursors.

Thirdly, primary particles have to be exposed enough to both precursors.“Exposure” is especially defined, in surface chemistry, as product ofpartial pressure of a reactant and the resident time to quantitativelycharacterize this process. Since it is, usually, not realistic for aprecursor to reside in the gas-fluidized bed for an extremely long timefrom the economic point of view even though a fluidized bed can have awide resident time distribution, it is, therefore, essential to maintaina high enough precursor partial pressure in the gas-fluidized bed. Assuch, a suitable precursor delivery system provides the precursoradequate exposure time for a good film growth, particularly as fluidizedpowders have significantly-large specific surface areas, as compared tothe cases for a traditional ALD coating process. Normally, the exposurefor a gas-fluidized bed should be no less than about 10⁶ Langmuir.Ideally, it should be in the range of about 10⁶ to 10⁸ L.

Preferably, a pre-heating of the primary particles is conducted toremove any volatile organic absorbents and free water. Optionally, ozonepre-treatment to initialize the functional group on powder surfaces canhelp film growth, said pretreatment done in situ, which is to say withinthe gas-fluidized bed reactor. Examples of these functional groupsinclude hydroxyls, (—OH), Halides (—X), amides (—NH3), —COOH, Alkyls(—R), alkoxides (—OR), and combinations thereof.

Coated Particle Detail

The invented process enables the formation of a coated particle that ispreferably uniform disperse after the coating material is deposited.“Uniform disperse” particles do not form significant amounts ofagglomerates during the process of coating the substrate particles withthe inorganic material. Particles are considered to be a uniformdisperse when (a) the average particle size does not increase more thanabout 5%, preferably not more than about 2%, more preferably not morethan about 1% as a result of the coating. The only increase of particlesize should be attributed to the coating itself. As such, formicro-scale particles (i.e., particles between about 1 and 100 micronsin diameter) particle size increases as a result of coating thicknessesare negligible.

A feature of the invention is that the depositing process occurs at amolecular scale in gas phase. This enables the reactants to diffuse intothe surfaces of individual particles. As such, those surfaces locatedinside very small pores of the primary particles also are coated.Consequently, despite the formation of these loose agglomerates, allsurfaces of the particles, both outside and inside, are fully coated.The result is that all individual particles, as opposed to agglomeratescomprised of particles, are coated. The invented method to provide ahighly uniform fluidization of the primary particles enables thiscomplete coating process.

When alumina was used as a coating for cathode particles in the ALDprotocol, capacity retention of the battery improved. FIG. 4 is aphotomicrograph of the alumina layer over LiNi_(1/3) Co_(1/3)Mn_(1/3)O₂spinel. Preferably, alumina coatings of less than 5 nm provide optimalresults, when voltages of more than about 3.5 V are utilized andtemperatures of about 55° C. exist.

An exemplary ALD-coating of ZrO₂ ultrathin film was fabricated using theinvented coherently-controlled gas-fluidized bed reactor. The number ofALD coating cycles varied, depending on the thickness of the zirconia.(A cycle is defined as a four-step sequential ALD process comprising tworeactant doses and two inert gas purges. Typically one cycle generatesabout a 0.08 to 0.15 nm thick film). In one coating experiment, 40cycles were used to generate about a 5 nm thick coating, while 80 cycleswere used to generate a 10 nm coating. The growth rate was approximately1.2 Angstroms per cycle, with the temperature maintained at about 200°C.

The SEM and TEM images, as shown in FIGS. 5A and 5B, for the coatedparticles indicate that an ALD-coated ZrO₂ ultrathin film on surface ofLiNi_(1/3)Co_(1/3)Mn_(1/3)O₂ cathode particles was obtained. The 4 nmfilm is amorphous, and highly conformal. For the coating depicted inFIGS. 5A and 5B, 50 cycles were utilized to generate the film, with 150grams of Li—Co—Ni—Mn—O particle material maintained at about 180° C. inthe gas-fluidized state.

In summary, the invention provides a gas-fluidization technique anddevice for homogeneously dispersing ultrafine powders (e.g., less than20 micrometers) in gas flows and stably fluidizing ultra fine powdersvia the utilization of standing waves in a gas-fluidized bed. Particlemasses of at least 200 grams per batch are regularly processed, with 1-5kilogram masses achievable with scale up.

FIG. 6 provides an element and weight profile analysis for theaforementioned zirconium-coated spinel. The ordinate on the graphdesignates weight percents of the various elements of the coatedconstruct. The abscissa on the graph designates the various positionsalong the construct for which the weight percents are found. The graphshows a relatively constant weight percentage for each of the elementsalong a distance of the coated substrate.

A standing wave in a 3-D deep gas-fluidized bed is generated by using awave reflector above the suspended powder bed in conjunction with amodulated gas flow.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (and/or aspects thereof) may be used in combination witheach other. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the inventionwithout departing from its scope. While the dimensions and types ofmaterials described herein are intended to define the parameters of theinvention, they are by no means limiting, but are instead exemplaryembodiments. Many other embodiments will be apparent to those of skillin the art upon reviewing the above description. The scope of theinvention should, therefore, be determined with reference to theappended claims, along with the full scope of equivalents to which suchclaims are entitled. In the appended claims, the terms “including” and“in which” are used as the plain-English equivalents of the terms“comprising” and “wherein.” Moreover, in the following claims, the terms“first,” “second,” and “third,” are used merely as labels, and are notintended to impose numerical requirements on their objects. Further, thelimitations of the following claims are not written inmeans-plus-function format and are not intended to be interpreted basedon 35 U.S.C. § 112, sixth paragraph, unless and until such claimlimitations expressly use the phrase “means for” followed by a statementof function void of further structure.

The present methods can involve any or all of the steps or conditionsdiscussed above in various combinations, as desired. Accordingly, itwill be readily apparent to the skilled artisan that in some of thedisclosed methods certain steps can be deleted or additional stepsperformed without affecting the viability of the methods.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as “up to,” “at least,” “greater than,” “less than,” “more than”and the like include the number recited and refer to ranges which can besubsequently broken down into subranges as discussed above. In the samemanner, all ratios disclosed herein also include all subratios fallingwithin the broader ratio.

One skilled in the art will also readily recognize that where membersare grouped together in a common manner, such as in a Markush group, thepresent invention encompasses not only the entire group listed as awhole, but each member of the group individually and all possiblesubgroups of the main group. Accordingly, for all purposes, the presentinvention encompasses not only the main group, but also the main groupabsent one or more of the group members. The present invention alsoenvisages the explicit exclusion of one or more of any of the groupmembers in the claimed invention.

The invention claimed is:
 1. A system for coating particles, the systemcomprising: a. a reaction zone; b. a means for producing fluidizedparticles within the reaction zone; c. a means for producing a standingfluid wave in the reaction zone by modulating fluid flow of a fluidizinggas; and d. a means for introducing coating moieties to the reactionzone.
 2. The system as recited in claim 1 wherein the reaction zonecontains an adjustable surface adapted to reflect the modulating fluidflow.
 3. The system as recited in claim 1 wherein the means forproducing a standing fluid wave causes the fluidizing fluid to contactthe fluidized particles to produce the standing wave.
 4. The system asrecited in claim 3 wherein the modulating fluid flow is coherentlyapplied to the fluidized particles.
 5. The system as recited in claim 1wherein the particles are no larger than 100 microns in diameter.
 6. Thesystem as recited in claim 1 wherein the coating moieties comprise atleast a portion of the standing fluid wave.
 7. The system as recited inclaim 1 wherein the particles are different sizes.
 8. The system asrecited in claim 1 wherein the reaction zone is defined by a reactionchamber and the reaction chamber remains motionless.
 9. The system asrecited in claim 1 wherein the means for introducing coating moieties tothe reaction zone includes a computer-controlled injections system witha fixed time interval between injections for alternatingly introducingtwo coating moieties into the reaction zone.